Vector: Add navigation collapsing feature

* Moved from the Vector extension (removed there in I47950b5375cae38e2).
* Removed bucket testing
* Removed "new" version of collapsible nav
  (the language portal splitting feature) as it didn't seem to be active
  on any wikis. Can be revived later if considered useful still.
* Killed the vector-collapsiblenav preference
** Usage on Wikimedia wikis:
  - 0.008% on en.wikipedia
  - 0.02% on de.wikipedia

Bug: 46512
Change-Id: I87149d7e15931f02ab700164e9e1d3d707b5e6a5